Datasheet

LM4917
www.ti.com SNAS238G –AUGUST 2004–REVISED MAY 2013
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input coupling capacitor (C
i
in Figure 2). A high value
capacitor can be expensive and may compromise space efficiency in portable designs. In many cases, however,
the speakers used in portable systems, whether internal or external, have little ability to reproduce signals below
150Hz. Applications using speakers with this limited frequency response reap little improvement by using high
value input and output capacitors.
Besides affecting system cost and size, C
i
has an effect on the LM4917's click and pop performance. The
magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by
selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency.
As shown in Figure 2, the input resistor, R
i
and the input capacitor, C
i
, produce a -3dB high pass filter cutoff
frequency that is found using Equation (3).
f
i-3dB
= 1 / 2πR
i
C
i
(4)
Also, careful consideration must be taken in selecting a certain type of capacitor to be used in the system.
Different types of capacitors (tantalum, electrolytic, ceramic) have unique performance characteristics and may
affect overall system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 90mW/16Ω Audio Amplifier
Given:
Power Output 90mW
Load Impedance 16Ω
Input Level 1Vrms (max)
Input Impedance 20kΩ
Bandwidth 100Hz–20kHz ± 0.50dB
The design begins by specifying the minimum supply voltage necessary to obtain the specified output power.
One way to find the minimum supply voltage is to use the Output Power vs Supply Voltage curve in the Typical
Performance Characteristics section. Another way, using Equation 5, is to calculate the peak output voltage
necessary to achieve the desired output power for a given load impedance. For a single-ended application, the
result is Equation 5.
(5)
V
DD
≥ [2V
OPEAK
+ (V
DOTOP
+ V
DOBOT
)] (6)
The Output Power vs Supply Voltage graph for a 16Ω load indicates a minimum supply voltage of 3.1V. This is
easily met by the commonly used 3.3V supply voltage. The additional voltage creates the benefit of headroom,
allowing the LM4917 to produce peak output power in excess of 90mW without clipping or other audible
distortion. The choice of supply voltage must also not create a situation that violates maximum power dissipation
as explained above in the POWER DISSIPATION section. Remember that the maximum power dissipation point
from Equation 2 must be multiplied by two since there are two independent amplifiers inside the package. Once
the power dissipation equations have been addressed, the required gain can be determined from Equation 7.
(7)
Thus, a minimum gain of 1.2 allows the LM4917 to reach full output swing and maintain low noise and THD+N
perfromance. For this example, let A
V
= 1.5.
The amplifiers overall gain is set using the input (R
i
) and feedback (R
f
) resistors. With the desired input
impedance set at 20kΩ, the feedback resistor is found using Equation (7).
A
V
= R
f
/ R
i
where
• The value of R
f
is 30kΩ (8)
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